New data from Denmark, one of the areas where Omicron spread the fastest, has shown that people can be reinfected by the virus, and specifically one of its subvariants.

It’s a rarity to be sure. Of the 1.8 million positive tests the researchers studied, just 47 people showed an infection with both the BA.1 and the BA.2 “stealth” strain, which spread at an even faster rate. Reinfections took place within a 20- to 60-day interval.

“In 47 of the cases, the affected individual first became infected by BA.1 and then by BA.2,” researchers said in a statement. “The majority of the infected were young and unvaccinated, and most experienced mild symptoms during their infections. The difference between the severity during their first and second infection was negligible. None of the infected individuals had become seriously ill, and none required admission to hospital.”

The remaining 20 people were likely infected by the same Omicron strain twice.

Since November, roughly 2 million people in Denmark have tested positive for Omicron. As in other regions of the world, BA.1 is the most common strain, but BA.2 is gaining ground quickly. On Feb. 1, however, the country declared COVID-19 was no longer a threat to society and lifted all restrictions.

The study has not been peer reviewed, but researchers have been looking at whether reinfection from the same strain was possible for the past two months, with some noting the immunocompromised are especially vulnerable.